From the American document U.S. Pat. No. 5,480,330, such a driving device is previously known which shows a device that is provided with two contra-rotating impellers, which provide a water flow through a water tunnel the rotations of which are provided by the fact that a first drive shaft drives a first impeller in a first direction of rotation and that a second drive shaft, concentric with the first drive shaft, drives a second impeller in a second direction of rotation by a toothed gearing being situated between the drive shafts.
As is seen in the American document U.S. Pat. No. 5,480,330, the toothed gearing, which provides the opposite directions of rotation, is situated outside the water tunnel, which implies that the two drive shafts will be relatively long at the same time as the toothed gearing will occupy no little volume beside the water tunnel.
Thus, such a driving device will be bulky in an installation in a boat on one hand by the gear case being placed outside the drive device as well as by the device requiring long double concentric drive shafts. By long concentric drive shafts, there is a great risk of unbalances and vibrations caused by the drive shafts.
The American document U.S. Pat. No. 6,273,768 shows a drive device for water operation which is provided with two contra-rotating impellers. The document shows that the drive device is provided with a conical toothed gearing to provide the contra-rotating motions of the impellers. The toothed gearing is placed inside the water tunnel and fitted in a gear housing placed downstream of the two impellers. The gear housing is in this connection stationary and fixedly connected to the inside of the water tunnel. A change gear of the toothed gearing is journalled in a fixed bearing housing directly in the water tunnel. Such a fixed bearing housing in the water tunnel will contribute to the water current through the drive device becoming turbulent by flow obstruction at these bearing housings. By the conical toothed gearing being formed with coaxial drive shafts, which extend outside the plane of rotation of the impellers, there is a great risk of vibrations and journalling problems for the drive shafts.